US 8194063 B2
Subpixels on an electroluminescent (EL) display panel, such as an organic light-emitting diode (OLED) panel, are compensated for initial nonuniformity (“mura”) and for aging effects such as threshold voltage Vth shift, EL voltage Voled shift, and OLED efficiency loss. The drive current of each subpixel is measured at one or more measurement reference gate voltages to form status signals representing the characteristics of the drive transistor and EL emitter of those subpixels. Current measurements are taken in the linear region of drive transistor operation to improve signal-to-noise ratio in systems such as modern LTPS PMOS OLED displays, which have relatively small Voled shift over their lifetimes and thus relatively small current change due to channel-length modulation. Various sources of noise are also suppressed to further increase signal-to-noise ratio.
1. Apparatus for providing drive transistor control signals to the gate electrodes of drive transistors in a plurality of EL subpixels in an EL panel, comprising a first voltage supply, a second voltage supply, and a plurality of EL subpixels in the EL panel, each EL subpixel comprising a drive transistor for applying current to an EL emitter in each EL subpixel, each drive transistor comprising a first supply electrode electrically connected to the first voltage supply and a second supply electrode electrically connected to a first electrode of the EL emitter; and each EL emitter comprising a second electrode electrically connected to the second voltage supply, the improvement comprising:
a sequence controller for selecting one or more of the plurality of EL subpixels;
a test voltage source electrically connected to the gate electrodes of the drive transistors of the one or more selected EL subpixels;
a voltage controller for controlling voltages of the first voltage supply, second voltage supply, and test voltage source to operate the drive transistors of the one or more selected EL subpixels in a linear region;
a measuring circuit for measuring the current passing through the first and second voltage supplies to provide respective status signals for each of the one or more selected EL subpixels representing the characteristics of the drive transistor and EL emitter of those subpixels, the current being measured while the drive transistors of the one or more selected EL subpixels are operated in the linear region;
means for providing a linear code value for each subpixel;
a compensator for changing the linear code values in response to the status signals to compensate for variations in the characteristics of the drive transistor and EL emitter in each subpixel; and
a source driver for producing the drive transistor control signals in response to the changed linear code values for driving the gate electrodes of the drive transistors.
2. The apparatus of
means for providing a respective target signal for each EL subpixel,
wherein the measuring circuit uses the target signals while providing the respective status signals for each of the one or more selected EL subpixels.
3. The apparatus of
4. The apparatus of
5. The apparatus of
each EL emitter comprises an OLED emitter; and
each drive transistor comprises a low temperature polysilicon transistor.
6. The apparatus of
a current to voltage converter for producing a voltage signal; and
a correlated double-sampling unit responsive to the voltage signal used in providing the status signal to the compensator.
7. The apparatus of
a plurality of second voltage supplies,
wherein the second electrode of each EL emitter comprises a electrically connected to only one second voltage supply.
8. The apparatus of
the plurality of EL subpixels in the EL panel are arranged in rows and columns; and
the sequence controller selects all EL subpixels in a selected row.
9. The apparatus of
10. The apparatus of
the measuring circuit measures the current passing through the first and second voltage supplies at different times; and
each status signal represents variations in the characteristics of the respective drive transistor and EL emitter caused by operation of the respective drive transistor and EL emitter over time.
11. The apparatus of
12. The apparatus of
13. The apparatus of
a first current mirror for producing a mirrored current which is a function of the drive current passing through the first and second supply electrodes; and
a second current mirror for applying a bias current to the first current mirror to reduce impedance of the first current mirror.
14. The apparatus of
Reference is made to commonly-assigned, co-pending U.S. patent application Ser. No. 11/962,182 filed Dec. 21, 2007, entitled “Electroluminescent Display Compensated Analog Transistor Drive Signal” to Leon et al, U.S. patent application Ser. No. 12/274,559 filed Nov. 20, 2008, entitled “Electroluminescent Display Initial-Nonuniformity-Compensated Drive Signal” to Leon et al. and U.S. patent application Ser. No. 12/396,662 filed Mar. 3, 2009, entitled “Electroluminescent Subpixel Compensated Drive Signal” by Levey et al, the disclosures of which are incorporated herein.
The present invention relates to control of a signal applied to a drive transistor for supplying current through a plurality of electroluminescent emitters on an electroluminescent display.
Flat-panel displays are of great interest as information displays for computing, entertainment, and communications. For example, electroluminescent (EL) emitters have been known for some years and have recently been used in commercial display devices. Such displays employ both active-matrix and passive-matrix control schemes and can employ a plurality of subpixels. Each subpixel contains an EL emitter and a drive transistor for driving current through the EL emitter. The subpixels are typically arranged in two-dimensional arrays with a row and a column address for each subpixel, and having a data value associated with the subpixel. Subpixels of different colors, such as red, green, blue, and white are grouped to form pixels. EL displays can be made using various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED).
Electroluminescent (EL) flat-panel display technologies, such as organic light-emitting diode (OLED) technology, provide benefits in color gamut, luminance, and power consumption over other technologies such as liquid-crystal display (LCD) and plasma display panel (PDP). However, EL displays suffer from performance degradation over time. In order to provide a high-quality image over the life of the display, this degradation must be compensated for. Furthermore, OLED displays suffer from visible nonuniformities across a display. These nonuniformities can be attributed to both the EL emitters in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the EL emitters.
The light output of an EL emitter is roughly proportional to the current through the emitter, so the drive transistor in an EL subpixel is typically configured as a voltage-controlled current source responsive to a gate-to-source voltage Vgs. Source drivers similar to those used in LCD displays provide the control voltages to the drive transistors. Source drivers can convert a desired code value into an analog voltage to control a drive transistor. The relationship between code value and voltage is typically non-linear, although linear source drivers with higher bit depths are becoming available. Although the nonlinear code value-to-voltage relationship has a different shape for OLEDs than the characteristic LCD S-shape (shown in e.g. U.S. Pat. No. 4,896,947), the source driver electronics required are very similar between the two technologies. In addition to the similarity between LCD and EL source drivers, LCD displays and EL displays are typically manufactured on the same substrate: amorphous silicon (a-Si), as taught e.g. by Tanaka et al. in U.S. Pat. No. 5,034,340. Amorphous Si is inexpensive and easy to process into large displays.
Amorphous silicon, however, is metastable: over time, as voltage bias is applied to the gate of an a-Si TFT, its threshold voltage (Vth) shifts, thus shifting its I-V curve (Kagan & Andry, ed. Thin-film Transistors. New York: Marcel Dekker, 2003. Sec. 3.5, pp. 121-131). Vth typically increases over time under forward bias, so over time, Vth shift will, on average, cause a display to dim.
In addition to a-Si TFT instability, modern EL emitters have their own instabilities. For example, in OLED emitters, over time, as current passes through an OLED emitter, its forward voltage (Voled) increases and its efficiency (typically measured in cd/A) decreases (Shinar, ed. Organic Light-Emitting Devices: a survey. New York: Springer-Verlag, 2004. Sec. 3.4, pp. 95-97). The loss of efficiency causes a display to dim on average over time, even when driven with a constant current. Additionally, in typical OLED display configurations, the OLED is attached to the source of the drive transistor. In this configuration, increases in Voled will increase the source voltage of the transistor, lowering Vgs and thus, the current through the OLED emitter (Ioled), and therefore causing dimming over time.
These three effects (Vth shift, OLED efficiency loss, and Voled rise) cause each individual OLED subpixel to lose luminance over time at a rate proportional to the current passing through that OLED subpixel. (Vth shift is the primary effect, Voled shift the secondary effect, and OLED efficiency loss the tertiary effect.) Therefore, as the display dims over time, those subpixels that are driven with more current will fade faster. This differential aging causes objectionable visible burn-in on displays. Differential aging is an increasing problem today as, for example, more and more broadcasters continuously superimpose their logos over their content in a fixed location. Typically, a logo is brighter than content around it, so the pixels in the logo age faster than the surrounding content, making a negative copy of the logo visible when watching content not containing the logo. Since logos typically contain high-spatial-frequency content (e.g. the AT&T globe), one subpixel can be heavily aged while an adjacent subpixel is only lightly aged. Therefore, each subpixel must be independently compensated for aging to eliminate objectionable visible burn-in.
Moreover, some transistor technologies, such as low-temperature polysilicon (LTPS), can produce drive transistors that have varying mobilities and threshold voltages across the surface of a display (Kuo, Yue, ed. Thin Film Transistors: Materials and Processes, vol. 2: Polycrystalline Thin Film Transistors. Boston: Kluwer Academic Publishers, 2004. pg. 412). This produces objectionable nonuniformity. Further, nonuniform OLED material deposition can produce emitters with varying efficiencies, also causing objectionable nonuniformity. These nonuniformities are present at the time the panel is sold to an end user, and so are termed initial nonuniformities, or “mura.”
It has been known to compensate for one or more of the three aging effects. Similarly, it is known in the prior art to measure the performance of each pixel in a display and then to correct for the performance of the pixel to provide a more uniform output across the display.
Considering Vth shift, the primary effect and one which is reversible with applied bias (Mohan et al., “Stability issues in digital circuits in amorphous silicon technology,” Electrical and Computer Engineering, 2001, Vol. 1, pp. 583-588), compensation schemes are generally divided into four groups: in-pixel compensation, in-pixel measurement, in-panel measurement, and reverse bias.
In-pixel Vth compensation schemes add additional circuitry to each subpixel to compensate for the Vth shift as it happens. For example, Lee et al., in “A New a-Si:H TFT Pixel Design Compensating Threshold Voltage Degradation of TFT and OLED”, SID 2004 Digest, pp. 264-274, teach a seven-transistor, one-capacitor (7T1C) subpixel circuit which compensates for Vth shift by storing the Vth of each subpixel on that subpixel's storage capacitor before applying the desired data voltage. Methods such as this compensate for Vth shift, but they cannot compensate for Voled rise or OLED efficiency loss. These methods require increased subpixel complexity and increased subpixel electronics size compared to the conventional 2T1C voltage-drive subpixel circuit. Increased subpixel complexity reduces yield, because the finer features required are more vulnerable to fabrication errors. Particularly in typical bottom-emitting configurations, increased total size of the subpixel electronics increases power consumption because it reduces the aperture ratio, the percentage of each subpixel which emits light. Light emission of an OLED is proportional to area at a fixed current, so an OLED emitter with a smaller aperture ratio requires more current to produce the same luminance as an OLED with a larger aperture ratio. Additionally, higher currents in smaller areas increase current density in the OLED emitter, which accelerates Voled rise and OLED efficiency loss.
In-pixel measurement Vth compensation schemes add additional circuitry to each subpixel to permit values representative of Vth shift to be measured. Off-panel circuitry then processes the measurements and adjusts the drive of each subpixel to compensate for Vth shift. For example, Nathan et al., in U.S. Patent Application Publication No. 2006/0273997, teach a four-transistor pixel circuit which permits TFT degradation data to be measured as either current under given voltage conditions or voltage under given current conditions. Nara et al., in U.S. Pat. No. 7,199,602, teach adding an inspection interconnect to a display, and adding a switching transistor to each pixel of the display to connect it to the inspection interconnect. Kimura et al., in U.S. Pat. No. 6,518,962, teach adding correction TFTs to each pixel of a display to compensate for EL degradation. These methods share the disadvantages of in-pixel Vth compensation schemes, but some can additionally compensate for Voled shift or OLED efficiency loss.
In-pixel measurement Vth compensation schemes add circuitry around a panel to take and process measurements without modifying the design of the panel. For example, Naugler et al., in U.S. Patent Application Publication No. 2008/0048951, teach measuring the current through an OLED emitter at various gate voltages of a drive transistor to locate a point on precalculated lookup tables used for compensation. However, this method requires a large number of lookup tables, consuming a significant amount of memory. Further, this method does not recognize the problem of integrating compensation with image processing typically performed in display drive electronics. It also does not recognize the limitations of typical display drive hardware, and so requires a timing scheme which is difficult to implement without expensive custom circuitry.
Reverse-bias Vth compensation schemes use some form of reverse voltage bias to shift Vth back to some starting point. These methods cannot compensate for Voled rise or OLED efficiency loss. For example, Lo et al., in U.S. Pat. No. 7,116,058, teach modulating the reference voltage of the storage capacitor in an active-matrix pixel circuit to reverse-bias the drive transistor between each frame. Applying reverse-bias within or between frames prevents visible artifacts, but reduces duty cycle and thus peak brightness. Reverse-bias methods can compensate for the average Vth shift of the panel with less increase in power consumption than in-pixel compensation methods, but they require more complicated external power supplies, can require additional pixel circuitry or signal lines, and may not compensate individual subpixels that are more heavily faded than others.
Considering Voled shift and OLED efficiency loss, U.S. Pat. No. 6,995,519 by Arnold et al. is one example of a method that compensates for aging of an OLED emitter. This method assumes that the entire change in emitter luminance is caused by changes in the OLED emitter. However, when the drive transistors in the circuit are formed from a-Si, this assumption is not valid, as the threshold voltage of the transistors also changes with use. The method of Arnold will thus not provide complete compensation for subpixel aging in circuits wherein transistors show aging effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate tracking/prediction of reverse bias effects, or a direct measurement of the OLED voltage change or transistor threshold voltage change.
Alternative methods for compensation measure the light output of each subpixel directly, as taught e.g. by Young et al. in U.S. Pat. No. 6,489,631. Such methods can compensate for changes in all three aging factors, but require either a very high-precision external light sensor, or integrated light sensors in each subpixel. An external light sensor adds to the cost and complexity of a device, while integrated light sensors increase subpixel complexity and electronics size, with attendant performance reductions.
Regarding initial-nonuniformity compensation, U.S. Patent Application Publication No. 2003/0122813 by Ishizuki et al. discloses a display panel driving device and driving method for providing high-quality images without irregular luminance. The light-emission drive current flowing is measured while each pixel successively and independently emits light. Then the luminance is corrected for each input pixel data based on the measured drive current values. According to another aspect, the drive voltage is adjusted such that one drive current value becomes equal to a predetermined reference current. In a further aspect, the current is measured while an off-set current, corresponding to a leak current of the display panel, is added to the current output from the drive voltage generator circuit, and the resultant current is supplied to each of the pixel portions. The measurement techniques are iterative, and therefore slow. Further, this technique is directed at compensation for aging, not for initial nonuniformity.
U.S. Pat. No. 6,081,073 by Salam describes a display matrix with a process and control means for reducing brightness variations in the pixels. This patent describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will lead to an overall reduction in the dynamic range and brightness of the display and a reduction and variation in the bit depth at which the pixels can be operated.
U.S. Pat. No. 6,473,065 by Fan describes methods of improving the display uniformity of an OLED. In this method, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, the described approaches require either a lookup table providing a complete characterization for each pixel, or extensive computational circuitry within a device controller. This is likely to be expensive and impractical in most applications.
U.S. Pat. No. 7,345,660 by Mizukoshi et al. describes an EL display having stored correction offsets and gains for each subpixel, and having a measurement circuit for measuring the current of each subpixel. While this apparatus can correct for initial nonuniformity, it uses a sense resistor to measure current, and thus has limited signal-to-noise performance. Furthermore, the measurements required by this method can be very time-consuming for large panels.
U.S. Pat. No. 6,414,661 by Shen et al. describes a method and associated system that compensates for long-term variations in the light-emitting efficiency of individual organic light emitting diodes in an OLED display device by calculating and predicting the decay in light output efficiency of each pixel based on the accumulated drive current applied to the pixel and derives a correction coefficient that is applied to the next drive current for each pixel. This patent describes the use of a camera to acquire images of a plurality of equal-sized sub-areas. Such a process is time-consuming and requires mechanical fixtures to acquire the plurality of sub-area images.
U.S. Patent Application Publication No. 2005/0007392 by Kasai et al. describes an electro-optical device that stabilizes display quality by performing correction processing corresponding to a plurality of disturbance factors. A grayscale characteristic generating unit generates conversion data having grayscale characteristics obtained by changing the grayscale characteristics of display data that defines the grayscales of pixels with reference to a conversion table whose description contents include correction factors. However, their method requires a large number of LUTs, not all of which are in use at any given time, to perform processing, and does not describe a method for populating those LUTs.
U.S. Pat. No. 6,989,636 by Cok et al. describes using a global and a local correction factor to compensate for nonuniformity. However, this method assumes a linear input and is consequently difficult to integrate with image-processing paths having nonlinear outputs.
U.S. Pat. No. 6,897,842 by Gu describes using a pulse width modulation (PWM) mechanism to controllably drive a display (e.g., a plurality of display elements forming an array of display elements). A non- uniform pulse interval clock is generated from a uniform pulse interval clock, and then used to modulate the width, and optionally the amplitude, of a drive signal to controllably drive one or more display elements of an array of display elements. A gamma correction is provided jointly with a compensation for initial nonuniformity. However, this technique is only applicable to passive-matrix displays, not to the higher-performance active-matrix displays which are commonly employed.
Existing mura and Vth compensation schemes are not without drawbacks, and few of them compensate for Voled rise or OLED efficiency loss. Those that compensate each subpixel for Vth shift do so at the cost of panel complexity and lower yield. There is a continuing need, therefore, for improving compensation to overcome these objections to compensate for EL panel degradation and prevent objectionable visible burn-in over the entire lifetime of an EL display panel, including at the start of its life.
In accordance with the present invention, there is provided, in apparatus for providing drive transistor control signals to the gate electrodes of drive transistors in a plurality of EL subpixels in an EL panel, including a first voltage supply, a second voltage supply, and a plurality of EL subpixels in the EL panel; each EL subpixel including a drive transistor for applying current to an EL emitter in each EL subpixel, each drive transistor having a first supply electrode electrically connected to the first voltage supply and a second supply electrode electrically connected to a first electrode of the EL emitter; and each EL emitter including a second electrode electrically connected to the second voltage supply, the improvement comprising:
(a) a sequence controller for selecting one or more of the plurality of EL subpixels;
(b) a test voltage source electrically connected to the gate electrodes of the drive transistors of the one or more selected EL subpixels;
(c) a voltage controller for controlling voltages of the first voltage supply, second voltage supply and test voltage source to operate the drive transistors of the one or more selected EL subpixels in a linear region;
(d) a measuring circuit for measuring the current passing through the first and second voltage supplies to provide respective status signals for each of the one or more selected EL subpixels representing the characteristics of the drive transistor and EL emitter of those subpixels, wherein the current is measured while the drive transistors of the one or more selected EL subpixels are operated in the linear region;
(e) means for providing a linear code value for each subpixel;
(f) a compensator for changing the linear code values in response to the status signals to compensate for variations in the characteristics of the drive transistor and EL emitter in each subpixel; and
(g) a source driver for producing the drive transistor control signals in response to the changed linear code values for driving the gate electrodes of the drive transistors.
The present invention provides an effective way of providing the drive transistor control signal. It requires only one measurement of each subpixel to perform compensation. It can be applied to any active-matrix backplane. The compensation of the control signal has been simplified by using a look-up table (LUT) to change signals from nonlinear to linear so compensation can be in linear voltage domain. It compensates for Vth shift, Voled shift, and OLED efficiency loss without requiring complex pixel circuitry or external measurement devices. It does not decrease the aperture ratio of a subpixel. It has no effect on the normal operation of the panel. It can raise yield of good panels by making objectionable initial nonuniformity invisible. Improved S/N (signal/noise) is obtained by taking measurements of the characteristics of the EL subpixel while operating in the linear region of transistor operation.
The present invention compensates for mura (initial nonuniformity) and degradation in the drive transistors and electroluminescent (EL) emitters of a plurality of subpixels on an active-matrix EL display panel, such as an organic light-emitting diode (OLED) panel. In one embodiment, it compensates for Vth shift, Voled shift, and OLED efficiency loss of all subpixels on an active-matrix OLED panel. A panel includes a plurality of pixels, each of which includes one or more subpixels. For example, each pixel might include a red, a green, and a blue subpixel. Each subpixel includes an EL emitter, which emits light, and surrounding electronics. A subpixel is the smallest addressable element of a panel.
The discussion to follow first considers the system as a whole. It then proceeds to the electrical details of a subpixel, followed by the electrical details for measuring one subpixel and the timing for measuring multiple subpixels. It next covers how the compensator uses measurements. Finally, it describes how this system is implemented in one embodiment, e.g. in a consumer product, from the factory to end-of-life.
A compensator 13 receives the linear code value, which can correspond to the particular light intensity commanded from the EL subpixel. As a result of variations in the drive transistor and EL emitter caused by mura and by operation of the drive transistor and EL emitter in the EL subpixel over time, the EL subpixel will generally not produce the commanded light intensity in response to the linear code value. The compensator 13 outputs a changed linear code value that will cause the EL subpixel to produce the commanded intensity, thereby compensating for variations in the characteristics of the drive transistor and EL emitter caused by operation of the drive transistor and EL emitter over time, and for variations in the characteristics of the drive transistor and EL emitter from subpixel to subpixel. The operation of the compensator will be discussed further in “Implementation,” below.
The changed linear code value from the compensator 13 is passed to a source driver 14 which can be a digital-to-analog converter. The source driver 14 produces a drive transistor control signal, which can be an analog voltage or current, or a digital signal such as a pulse-width-modulated waveform, in response to the changed linear code value. In a preferred embodiment, the source driver 14 can be a source driver having a linear input-output relationship, or a conventional LCD or OLED source driver with its gamma voltages set to produce an approximately linear output. In the latter case, any deviations from linearity will affect the quality of the results. The source driver 14 can also be a time-division (digital-drive) source driver, as taught e.g. in commonly assigned WO 2005/116971 by Kawabe. The analog voltage from a digital-drive source driver is set at a predetermined level commanding light output for an amount of time dependent on the output signal from the compensator. A conventional source driver, by contrast, provides an analog voltage at a level dependent on the output signal from the compensator for a fixed amount of time (generally the entire frame). A source driver can output one or more drive transistor control signals simultaneously. A panel preferably has a plurality of source drivers, each outputting the drive transistor control signal for one subpixel at a time.
The drive transistor control signal produced by the source driver 14 is provided to an EL subpixel 15. This circuit, as will be discussed in “Display element description,” below. When the analog voltage is provided to the gate electrode of the drive transistor in the EL subpixel 15, current flows through the drive transistor and EL emitter, causing the EL emitter to emit light. There is generally a linear relationship between current through the EL emitter and luminance of the light output of the emitter, and a nonlinear relationship between voltage applied to the drive transistor and current through the EL emitter. The total amount of light emitted by an EL emitter during a frame can thus be a nonlinear function of the voltage from the source driver 14.
The current flowing through the EL subpixel is measured under specific drive conditions by a current-measurement circuit 16, as will be discussed further in “Data collection,” below. The measured current for the EL subpixel provides the compensator with the information it needs to adjust the commanded drive signal. This will be discussed further in “Algorithm,” below.
Display Element Description
First supply electrode 204 is electrically connected to first voltage supply 211 through PVDD bus line 1011, second electrode 208 is electrically connected to second voltage supply 206 through a sheet cathode 1012, and the drive transistor control signal is provided to gate electrode 203 by a source driver 14 across a column line e.g. 32 a when select transistor 36 is activated by a gate line 34.
Neglecting leakage, the same current, the drive current, passes from first voltage supply 211, through the first supply electrode 204 and the second supply electrode 205, through the EL emitter electrodes 207 and 208, to the second voltage supply 206. The drive current is what causes the EL emitter to emit light. Therefore, current can be measured at any point in this drive current path. Current can be measured off the EL panel at the first voltage supply 211 to reduce the complexity of the EL subpixel. Drive current is referred to herein as Ids, the current through the drain and source terminals of the drive transistor.
Still referring to
Each EL subpixel 15 is measured at a current corresponding to a measurement reference gate voltage (
The current mirror unit 210 is attached to voltage supply 211, although it can be attached anywhere in the drive current path. A first current mirror 212 supplies drive current to the EL subpixel 15 through a switch 200, and produces a mirrored current on its output 213. The mirrored current can be equal to the drive current, or a function of the drive current. For example, the mirrored current can be a multiple of the drive current to provide additional measurement-system gain. A second current mirror 214 and a bias supply 215 apply a bias current to the first current mirror 212 to reduce the impedance of the first current mirror viewed from the panel, advantageously increasing the response speed of the measurement circuit. This circuit also reduces changes in the current through the EL subpixels being measured due to voltage changes in the current mirror resulting from current draw of the measurement circuit. This advantageously improves signal-to-noise ratio over other current-measurement options, such as a simple sense resistor, which can change voltages at the drive transistor terminals depending on current. Finally, a current-to-voltage (I-to-V) converter 216 converts the mirrored current from the first current mirror into a voltage signal for further processing. The I-to-V converter 216 can include a transimpedance amplifier or a low-pass filter.
Switch 200, which can be a relay or FET, can selectively electrically connect the measuring circuit to the drive current flow through the first and second electrodes of the drive transistor 201. During measurement, the switch 200 can electrically connect first voltage supply 211 to first current mirror 212 to permit measurements. During normal operation, the switch 200 can electrically connect first voltage supply 211 directly to first supply electrode 204 rather than to first current mirror 212, thus removing the measuring circuit from the drive current flow. This causes the measurement circuitry to have no effect on normal operation of the panel. It also advantageously permits the measurement circuit's components, such as the transistors in the current mirrors 212 and 214, to be sized only for measurement currents and not for operational currents. As normal operation generally draws much more current than measurement, this permits substantial reduction in the size and cost of the measurement circuit.
The current mirror unit 210 permits measurement of the current for one EL subpixel at a time. To measure the current for multiple subpixels, in one embodiment the present invention uses correlated double-sampling, with a timing scheme usable with standard OLED source drivers.
In a standard timing sequence used in typical operation of this panel, the source driver 14 drives appropriate drive transistor control signals on the respective column lines 32 a, 32 b, 32 c. The gate driver 33 then activates the first row line 34 a, causing the appropriate control signals to pass through the select transistors 36 to the gate electrodes 203 of the appropriate drive transistors 201 to cause those transistors to apply current to their attached EL emitters 202. The gate driver 33 then deactivates the first row line 34 a, preventing control signals for other rows from corrupting the values passed through the select transistors 36. The source driver 14 drives control signals for the next row on the column lines 32 a, 32 b, 32 c, and the gate driver 33 activates the next row 34 b. This process repeats for all rows. In this way all EL subpixels 15 on the panel receive appropriate control signals, one row at a time. The row time is the time between activating one row line (e.g. 34 a) and activating the next (e.g. 34 b). This time is generally constant for all rows. A sequence controller 37 controls the source driver and gate driver appropriately to produce the standard timing sequence and provide appropriate data to each subpixel. The sequence controller also selects one or more of the plurality of EL subpixels 15 for measurement. The functions of the sequence controller and compensator can be provided in a single microprocessor or integrated circuit, or in separate devices.
According to the present invention, the sequence controller uses the standard timing sequence advantageously to select only one subpixel at a time, working down a column. Referring to
Referring now to
Referring back to
In an embodiment of the present invention, the sequence controller 37 can select one row of subpixels at a time, and the respective currents can be measured for each of the plurality of subpixels in the row using multiple measurement circuits, or a multiplexer connecting a single measurement circuit in turn to the drive current path through each subpixel. In another embodiment, the sequence controller can divide the subpixels on the panel into groups, and select different groups at different times. Each group can include e.g. only a subset of the subpixels in each column. This permits measurements to be taken more quickly, at the expense of not updating every subpixel's respective measurement each time a measurement is taken. In either embodiment, while measurements are taken, the test voltage source can provide drive transistor control signals only to the selected subpixels. The test voltage source can also provide to the selected subpixels drive transistor control signals causing significant drive current to flow, and to all subpixels not selected drive transistor control signals causing no current, or only dark current, to flow.
The analog or digital output of differential amplifier 223 can be provided directly to compensator 13. Alternatively, analog-to-digital converter 230 can preferably digitize the output of differential amplifier 223 to provide digital measurement data to compensator 13.
The measuring circuit 16 can preferably include a status signal generation unit 240 which receives the respective outputs of differential amplifier 223 and performs further processing to provide the respective status signals for each EL subpixel. Status signals can be digital or analog. Referring to
In a first embodiment of the present invention, each current difference, e.g. 43, can be the status signal for a corresponding subpixel. For example, current difference 43 can be the status signal for the subpixel attached to row line 34 b and column line 32 a. In this embodiment the status signal generation unit 240 can perform a linear transform on current differences, or pass them through unmodified. All subpixels can be measured at the same measurement reference gate voltage, so that the current (43) through each subpixel at the measurement reference gate voltage meaningfully represents the characteristics of the drive transistor and EL emitter in that subpixel. The current differences 43 can be stored in memory 619.
In a second embodiment, memory 619 stores a respective target signal i0 611 for each EL subpixel. Memory 619 also stores a most recent current measurement i1 612 of each EL subpixel, which can be the value most recently measured by the measurement circuit for the corresponding subpixel. Measurement 612 can also be an average of a number of measurements, an exponentially-weighted moving average of measurements over time, or the result of other smoothing methods which will be obvious to those skilled in the art. Target signal i0 611 and current measurement i1 612 can be compared as described below to provide a percent current 613, which can be the status signal for the EL subpixel. The target signal for a subpixel can be a current measurement of that subpixel taken at a different time than measurement i1 612, preferably before i1, and thus percent current can represent variations in the characteristics of the respective drive transistor and EL emitter caused by operation of the respective drive transistor and EL emitter over time. The target signal for a subpixel can also be a selected reference signal so that percent current represents the characteristics of the drive transistor and EL emitter in the respective EL subpixel at a particular time, and specifically with respect to the target.
In a third embodiment, memory 619 stores a mura-compensation gain term mg 615, and a mura-compensation offset term mo 616, calculated as described below. The status signal for each EL subpixel can include a respective gain and offset, and specifically respective mg and mo values. Values mg and mo are computed with respect to a target and thus represent variations in the characteristics of the respective drive transistors and EL emitters across multiple subpixels. Additionally, any (mg, mo) pair by itself represents the characteristics of the drive transistor and EL emitter in the respective subpixel.
These three embodiments can be used together. For example, the status signal for each subpixel can include percent current, mg and mo. Compensation, described below in “Implementation,” can be performed in the same way whether the status signal indicates variations for a single subpixel over time (aging) or variations across multiple subpixels at a particular time (mura). Memory 619 can include RAM, nonvolatile RAM, such as a Flash memory, and ROM, such as EEPROM. In one embodiment, the i0, mg and mo values are stored in EEPROM and the i1 values are stored in Flash.
Sources of Noise
In practice, the current waveform can be other than a clean step, so measurements can be taken only after waiting for the waveform to settle. Multiple measurements of each subpixel can also be taken and averaged together. Such measurements can be taken consecutively before advancing to the next subpixel. Such measurements can also be taken in separate measurement passes, in which each subpixel on the panel is measured in each pass. Capacitance between voltage supplies 206 and 211 can add to the settling time. This capacitance can be intrinsic to the panel or provided by external capacitors, as is common in normal operation. It can be advantageous to provide a switch that can be used to electrically disconnect the external capacitors while taking measurements.
Noise on any voltage supply will affect the current measurement. For example, noise on the voltage supply which the gate driver uses to deactivate rows (often called VGL or Voff, and typically around −8 VDC) can capacitively couple across the select transistor into the drive transistor and affect the current, thus making current measurements noisier. If a panel has multiple power-supply regions, for example a split supply plane, those regions can be measured in parallel. Such measurement can isolate noise between regions and reduce measurement time.
Whenever the source driver switches, its noise transients can couple into the voltage supply planes and the individual subpixels, causing measurement noise. To reduce this noise, the control signals out of the source driver can be held constant while stepping down a column. For example, when measuring a column of red subpixels on an RGB stripe panel, the red code value supplied to the source driver for that column can be constant for the entire column. This will eliminate source-driver transient noise.
Source driver transients can be unavoidable at the beginning and ends of columns, as the source driver has to change from activating the present column (e.g. 32 a) to activating the next column (e.g. 32 b). Consequently, measurements for the first and last one or more subpixels in any column can be subject to noise due to transients. In one embodiment, the EL panel can have extra rows, not visible to the user, above and below the visible rows. There can be enough extra rows that the source driver transients occur only in those extra rows, so measurements of visible subpixels do not suffer. In another embodiment, a delay can be inserted between the source driver transient at the beginning of a column and the measurement of the first row in that column, and between the measurement of the last row in that column and the source driver transient at the end of a column.
This discussion so far assumes that once a subpixel is turned on and settles to some current, it remains at that current for the remainder of the column. Two effects that can violate that assumption are storage-capacitor leaking and within-subpixel effects.
Even when the storage capacitor's value is stable, within-subpixel effects can corrupt measurements. A common within-subpixel effect is self-heating of the subpixel, which can change the current drawn by the subpixel over time. The drift mobility of an a-Si TFT is a function of temperature; increasing temperature increases mobility (Kagan & Andry, op. cit., sec. 2.2.2, pp. 42-43). As current flows through the drive transistor, power dissipation in the drive transistor and in the EL emitter will heat the subpixel, increasing the temperature of the transistor and thus its mobility. Additionally, heat lowers Voled; in cases where the OLED is attached to the source terminal of the drive transistor, this can increase Vgs of the drive transistor. These effects increase the amount of current flowing through the transistor. Under normal operation, self-heating can be a minor effect, as the panel can stabilize to an average temperature based on the average contents of the image it is displaying. However, when measuring subpixel currents, self-heating can corrupt measurements.
To correct for self-heating effects and any other within-subpixel effects producing similar noise signatures, the self-heating can be characterized and subtracted off the known self-heating component of each subpixel. Each subpixel generally increases current by the same amount during each row time, so with each succeeding subpixel the self-heating for all active subpixels can be subtracted off. For example, to calculate subpixel 3's current 424, measurement 423 can be reduced by self-heating amount 422, which is twice self-heating amount 421: amount 421 per subpixel, times two subpixels already active. The self-heating can be characterized by turning on one subpixel for tens or hundreds of row times and measuring its current periodically while it is on. The average slope of the current with respect to time can be multiplied by one row time to calculate the rise per subpixel per row time, i.e. self-heating amount 421.
Error due to self-heating, and power dissipation, can be reduced by selecting a lower measurement reference gate voltage (
At the measurement reference gate voltage 510, the un-aged subpixel produced the current represented at point 511. The aged sub-pixel, however, produces at that gate voltage the lower amount of current represented at point 512 a. Points 511 and 512 a can be two measurements of the same subpixel taken at different times. For example, point 511 can be a measurement at manufacturing time, and point 512 a can be a measurement after some use by a customer. The current represented at point 512 a would have been produced by the un-aged subpixel when driven with voltage 513 (point 512 b), so a voltage shift ΔVth 514 is calculated as the voltage difference between voltages 510 and 513. Voltage shift 514 is thus the shift required to bring the aged curve back to the un-aged curve. In this example, ΔVth 514 is just under two volts. Then, to compensate for the Vth shift, and drive the aged subpixel to the same current as the un-aged subpixel had, voltage shift 514 is added to every commanded drive voltage (linear code value). For further processing, percent current is also calculated as current 512 a divided by current 511. An unaged subpixel will thus have 100% current. Percent current is used in several algorithms according to the present invention. Any negative current reading 511, such as might be caused by extreme environmental noise, can be clipped to 0, or disregarded. Note that percent current is always calculated at the measurement reference gate voltage 510.
In general, the current of an aged subpixel can be higher or lower than that of an un-aged subpixel. For example, higher temperatures cause more current to flow, so a lightly-aged subpixel in a hot environment can draw more current than an unaged subpixel in a cold environment. The compensation algorithm of the present invention can handle either case; ΔVth 514 can be positive or negative (or zero, for unaged pixels). Similarly, percent current can be greater or less than 100% (or exactly 100%, for unaged pixels).
Since the voltage difference due to Vth shift is the same at all currents, any single point on the I-V curve can be measured to determine that difference. In one embodiment, measurements are taken at high gate voltages, advantageously increasing signal-to-noise ratio of the measurements, but any gate voltage on the curve can be used.
Voled shift is the secondary aging effect. As the EL emitter is operated, Voled shifts, causing the aged I-V curve to no longer be a simple shift of the un-aged curve. This is because Voled rises nonlinearly with current, so Voled shift will affect high currents differently than low currents. This effect causes the I-V curve to stretch horizontally as well as shifting. To compensate for Voled shift, two measurements at different drive levels can be taken to determine how much the curve has stretched, or the typical Voled shift of OLEDs under load can be characterized to permit estimation of Voled contribution in an open-loop manner. Both can produce acceptable results.
In one embodiment, the EL emitter 202 (
In a preferred embodiment, the EL emitter 202 is connected to the drain terminal of the drive transistor 201, for example, in PMOS non-inverted configurations, in which the OLED anode is tied to the drive transistor drain. Voled rise changes thus Vds of the drive transistor 201, as the OLED is connected in series with the drain-source path of the drive transistor. Modern OLED emitters, however, have much smaller ΔVoled than older emitters for a given amount of aging, reducing the magnitude of Vds change and thus of Ids change.
The requirement for extra measurement sensitivity can be mitigated by operating the drive transistor in the linear region of operation while taking current measurements. As is known in the electronics art, thin-film transistors conduct appreciable current in two different modes of operation: linear (Vds<Vgs−Vth) and saturation (Vds>=Vgs−Vth) (Lurch, op. cit., p. 111). In EL applications, the drive transistors are typically operated in the saturation region to reduce the effect of Vds variation on current. However, in the linear region of operation, where
In one embodiment of the present invention, therefore, the sequence controller 37 can include a voltage controller. While measuring currents as described above, the voltage controller can control voltages for the first voltage supply 211 and second voltage supply 206, and the drive transistor control signal from source driver 14 operating as a test voltage source, to operate drive transistor 201 in the linear region. For example, in a PMOS non-inverted configuration, the voltage controller can hold the PVDD voltage and the drive transistor control signal at constant values and increase the Vcom voltage to reduce Vds without reducing Vgs. When Vds falls below Vgs−Vth, the drive transistor will be operating in the linear region and a measurement can be taken.
The voltage controller can also be provided separately from the sequence controller as long as the two are coordinated to operate the transistors in the linear region during measurements. In an embodiment described above, in which the sequence controller selects different groups of EL subpixels at different times, the voltage controller can control the voltages for the PVDD supply 211 and Vcom supply 206, and the respective drive transistor control signals from source driver 14, to operate the drive transistor 201 in each selected EL subpixel in the linear region. A panel can have multiple PVDD and Vcom supplies, in which case each supply can be controlled independently according to which EL subpixels are selected to operate the drive transistor 201 in each selected EL subpixel in the linear region.
OLED efficiency loss is the tertiary aging effect. As an OLED ages, its efficiency decreases, and the same amount of current no longer produces the same amount of light. To compensate for this without requiring optical sensors or additional electronics, OLED efficiency loss as a function of Vth shift can be characterized, permitting estimation of the amount of extra current required to return the light output to its previous level. OLED efficiency loss can be characterized by driving an instrumented OLED subpixel with a typical input signal for a long period of time, and periodically measuring Vth, Voled and Ids at various drive levels. Efficiency can be calculated as Ids/Voled, and that calculation can be correlated to Vth or percent current. Note that this characterization achieves most effective results when Vth shift is always forward, since Vth shift can be reversed more simply than OLED efficiency loss. If Vth shift is reversed, correlating OLED efficiency loss with Vth shift can become complicated. For further processing, percent efficiency can be calculated as aged efficiency divided by new efficiency, analogously to the calculation of percent current described above.
To compensate for Vth and Voled shift and OLED efficiency loss due to operation of the drive transistor and EL emitter over time, the second above embodiment of the status signal generation unit 240 can be used. Subpixel currents can be measured at the measurement reference gate voltage 510. Un-aged current at point 511 is target signal i0 611. The most recent aged-subpixel current measurement 512 a is most recent current measurement i1 612. Percent current 613 is the status signal. Percent current 613 can be 0 (dead pixel), 1 (no change), less than 1 (current loss) or greater than 1 (current gain). Generally it will be between 0 and 1, because the most recent current measurement will be lower than the target signal, which can preferably be a current measurement taken at panel manufacturing time.
The second above embodiment of the status signal generation unit 240 can also be used to compensate for mura: differences in the characteristics of a plurality of OLED subpixels on a panel before aging. Referring back to
The third above embodiment of the status signal generation unit 240 can also be used in an embodiment for mura compensation. The current of each EL subpixel can be measured at a first and a second measurement reference gate voltage, or in general at a plurality of measurement reference gate voltages, to produce an I-V curve for each subpixel. A reference I-V curve can be calculated as the mean of all I-V curves, their minimum, or another mathematical function as will be obvious to those skilled in the art. A mura-compensation gain term mg 615 (
The reference I-V curve can be calculated as the mean of the I-V curves of all subpixel on the panel, or of the subpixels in a particular region of the panel. Multiple reference I-V curves can be provided for different regions of the panel or for different color channels.
The inputs to compensator 13 are the location 601 of an EL subpixel and a linear code value 602 of that subpixel. The linear code value 602 can represent a commanded drive voltage. The compensator 13 changes the linear code value 602 to produce a changed linear code value for a source driver, which can be e.g. a compensated voltage out 603. The compensator 13 can include four major blocks: determining a subpixel's age 61, optionally compensating for OLED efficiency 62, determining the compensation based on age 63, and compensating 64. Blocks 61 and 62 are primarily related to OLED efficiency compensation, and blocks 63 and 64 are primarily related to voltage compensation, specifically Vth/Voled compensation.
Percent current 613 is sent to the next processing stage 63, and is also input to a model 695 to determine the percent OLED efficiency 614. Model 695 outputs an efficiency 614 which is the amount of light emitted for a given current at the time of the most recent measurement, divided by the amount of light emitted for that current at manufacturing time. Any percent current greater than 1 can yield an efficiency of 1, or no loss, since efficiency loss can be difficult to calculate for pixels which have gained current. Model 695 can also be a function of the linear code value 602, as indicated by the dashed arrow, in cases where OLED efficiency depends on commanded current. Whether to include linear code value 602 as an input to model 695 can be determined by life testing and modeling of a panel design.
Referring back to
In parallel, the compensator receives a linear code value 602, e.g. a commanded voltage in. This linear code value 602 is passed through the original I-V curve 691 of the panel measured at manufacturing time to determine the desired current 621. This is divided by the percent efficiency 614 in operation 628 to return the light output for the desired current to its manufacturing-time value. The resulting, boosted current is then passed through curve 692, the inverse of curve 691, to determine what commanded voltage will produce the amount of light desired in the presence of efficiency loss. The value out of curve 692 is passed to the next stage as efficiency-adjusted voltage 622.
If efficiency compensation is not desired, linear code value 602 is sent unchanged to the next stage as efficiency-adjusted voltage 622, as indicated by optional bypass path 626. The percent current 613 is still calculated even if efficiency compensation is not desired, but the percent efficiency 614 need not be.
For straight Vth shift, α will be zero, and operation 633 will reduce to adding the Vth shift amount to the efficiency-adjusted voltage 622. For any particular subpixel, the amount to add is constant until new measurements are taken. Therefore, the voltage to add in operation 633 can be pre-computed after measurements are taken, permitting blocks 63 and 64 to collapse to looking up the stored value and adding it. This can save considerable logic.
Cross-Domain Processing, and Bit Depth
Image-processing paths known in the art typically produce nonlinear code values (NLCVs), that is, digital values having a nonlinear relationship to luminance (Giorgianni & Madden. Digital Color Management: encoding solutions. Reading, Mass.: Addison-Wesley, 1998. Ch. 13, pp. 283-295). Using nonlinear outputs matches the input domain of a typical source driver, and matches the code value precision range to the human eye's precision range. However, Vth shift is a voltage-domain operation, and thus is preferably implemented in a linear-voltage space. A source driver 14 can be used, and domain conversion performed before the source driver 14, to effectively integrate a nonlinear-domain image-processing path with a linear-domain compensator. Note that this discussion is in terms of digital processing, but analogous processing can be performed in an analog or mixed digital/analog system. Note also that the compensator can operate in linear spaces other than voltage. For example, the compensator can operate in a linear current space.
Referring to Quadrant I, domain-conversion unit 12 receives respective NLCVs for each subpixel, and converts them to LCVs. This conversion should be performed with sufficient resolution to avoid objectionable visible artifacts such as contouring and crushed blacks. In digital systems, NLCV axis 701 can be quantized, as indicated in
Transform 711 is an ideal transform for an unaged subpixel. It has no relationship to aging of any subpixel or the panel as a whole. Specifically, transform 711 is not modified due to any Vth, Voled, or OLED efficiency changes. There can be one transform for all colors, or one transform for each color. The domain-conversion unit, through transform 711, advantageously decouples the image-processing path from the compensator, permitting the two to operate together without having to share information. This simplifies the implementation of both. Domain-conversion unit 12 can be implemented as a look-up table or a function analogous to an LCD source driver.
Referring to Quadrant II, compensator 13 changes LCVs to changed linear code values (CLCVs) on a per-subpixel basis.
Transform 721 represents the compensator's behavior for an unaged subpixel. The CLCV can thus be the same as the LCV. Transform 722 represents the compensator's behavior for an aged subpixel. The CLCV can be the LCV plus an offset representing the Vth shift of the subpixel in question. Consequently, the CLCVs will generally require a larger range than the LCVs in order to provide headroom for compensation. For example, if a subpixel requires 256 LCVs when it is new, and the maximum shift over its lifetime is 128 LCVs, the CLCVs will need to be able to represent values up to 384=256+128 to avoid clipping the compensation of heavily-aged subpixels.
In one embodiment, the NLCVs from the image-processing path are nine bits wide. The LCVs are 11 bits wide. The transformation from nonlinear input signals to linear code values can be performed by a LUT or function. The compensator can take in the 11-bit linear code value representing the desired voltage and produce a 12-bit changed linear code value to send to a source driver 14. The source driver 14 can then drive the gate electrode of the drive transistor of an attached EL subpixel in response to the changed linear code value. The compensator can have greater bit depth on its output than its input to provide headroom for compensation, that is, to extend the voltage range 78 to voltage range 79 and simultaneously keep the same resolution across the new, expanded range, as required for minimum linear code value step 713. The compensator output range can extend below the range of transform 721 as well as above it.
Each panel design can be characterized to determine what the maximum Vth shift 73, Voled rise and efficiency loss will be over the design life of a panel, and the compensator 13 and source drivers 14 can have enough range to compensate. This characterization can proceed from required current to required gate bias and transistor dimensions via the standard transistor saturation-region Ids equation, then to Vth shift over time via various models known in the art for a-Si degradation over time.
Sequence of Operations
Panel Design Characterization
This section is written in the context of mass-production of a particular OLED panel design. Before mass-production begins, the design can be characterized: accelerated life testing can be performed, and I-V curves are measured for various subpixels of various colors on various sample panels aged to various levels. The number and type of measurements required, and of aging levels, depend on the characteristics of the particular panel. With these measurements, a value alpha (α) can be calculated and a measurement reference gate voltage can be selected. Alpha (
The α value can be calculated by optimization. An example is given in Table 1. ΔVth can be measured at a number of gate voltages, under a number of aging conditions. ΔVth differences are then calculated between each ΔVth and the ΔVth at the measurement reference gate voltage 510. Vg differences are calculated between each gate voltage and the measurement reference gate voltage 510. The inner term of Eq. 1, ΔVth·α·(Vg,ref−Vin), can then be computed for each measurement to yield a predicted ΔVth difference, using the appropriate ΔVth at the measurement reference gate voltage 510 as ΔVth in the equation, and using the appropriate calculated gate voltage difference as (Vg,ref−Vin). The α value can then be selected iteratively to reduce, and preferably mathematically minimize, the error between the predicted ΔVth differences and the calculated ΔVth differences. Error can be expressed as the maximum difference or the RMS difference. Alternative methods known in the art, such as least-squares fitting of ΔVth difference as a function of Vg difference, can also be used.
In addition to α and the measurement reference gate voltage, characterization can also determine, as described above, Voled shift as a function of Vth shift, efficiency loss as a function of Vth shift, self-heating component per subpixel, maximum Vth shift, Voled shift and efficiency loss, and resolution required in the nonlinear-to-linear transform and in the compensator. Resolution required can be characterized in conjunction with a panel calibration procedure such as co-pending commonly-assigned U.S. Patent Application Publication No. 2008/0252653, the disclosure of which is incorporated herein. Characterization also determines, as will be described in “In the field,” below, the conditions for taking characterization measurements in the field, and which embodiment of the status signal generation unit 240 to employ for a particular panel design. All these determinations can be made by those skilled in the art.
Once the design has been characterized, mass-production can begin. At manufacturing time, appropriate values are measured for each panel produced according to a selected embodiment of the status signal generation unit 240. For example, I-V curves and subpixel currents can be measured. I-V curves can be averages of curves for multiple subpixels. There can be separate curves for different colors, or for different regions of the panel. Current can be measured at enough drive voltages to make a realistic I-V curve; any errors in the I-V curve can affect the results. Subpixel currents can be measured at the measurement reference gate voltage to provide target signals i0 611. For mura compensation, two measurements are taken, and mg and mo values calculated, for each subpixel. The I-V curves, reference currents and mura-compensation values are stored in a nonvolatile memory associated with the panel and it is sent into the field.
In the Field
Once in the field, the subpixels on the panel age at different rates depending on how hard they are driven. After some time one or more pixels have shifted far enough that they need to be compensated; how to determine that time is considered below.
To compensate, compensation measurements are taken and applied. The compensation measurements are of the current of each subpixel at the measurement reference gate voltage. The measurements are applied as described in “Algorithm,” above. The measurements are stored so they can be applied whenever that subpixel is driven, until the next time measurements are taken. The sequence controller 37 can select the entire panel or any subset thereof when taking compensation measurements; when driving any subpixel, the most recent measurements for that subpixel can be used in the compensation. Status signals from the subpixels most recently measured can also be interpolated to estimate updated status signals for subpixels not measured in the most recent measurement pass. A first subset of the subpixels can thus be measured at one time and second subset at another time, permitting compensation across the panel even if not every subpixel has been measured in the most recent pass. Blocks larger than one subpixel can also be measured, and the same compensation applied to every subpixel in the block, but doing so requires care to avoid introducing block-boundary artifacts. Additionally, measuring blocks larger than one subpixel introduces vulnerability to visible burn-in of high spatial-frequency patterns; such patterns can have features smaller than the block size. This vulnerability can be traded off against the decreased time required to measure multiple-subpixel blocks compared to individual subpixels.
Compensation measurements can be taken as frequently or infrequently as desired; a typical range can be once every eight hours to once every four weeks.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
For example, the EL subpixel 15 shown in
In a preferred embodiment, the invention is employed in a display panel that includes Organic Light Emitting Diodes (OLEDs) which are composed of small molecule or polymeric OLEDs as disclosed in but not limited to U.S. Pat. No. 4,769,292, by Tang et al., and U.S. Pat. No. 5,061,569, by VanSlyke et al. Many combinations and variations of organic light emitting materials can be used to fabricate such a panel. Referring to
The above embodiments can apply to any active matrix backplane that is not stable as a function of time (such as a-Si), or that exhibits initial nonuniformity. For instance, transistors formed from organic semiconductor materials and zinc oxide are known to vary as a function of time and therefore this same approach can be applied to these transistors. Furthermore, as the present invention can compensate for EL emitter aging independently of transistor aging, this invention can also be applied to an active-matrix backplane with transistors that do not age, such as low-temperature poly-silicon (LTPS) TFTs. On an LTPS backplane, the drive transistor 201 and select transistor 36 are low-temperature polysilicon transistors.